1. Field of the Invention
The present invention relates to a ceramic honeycomb structure used as a catalyst carrier in a catalytic converter for cleaning exhaust gases from internal combustion engines and to a die for extruding the honeycomb structure.
2. Description of the Related Art
A ceramic honeycomb structure such as of cordierite has heretofore been used as a catalyst carrier in a catalytic converter for cleaning gases emitted from internal combustion engines such as automobile engines (see, for example, Japanese Examined Patent Publication (Kokoku) No. 61-47135). In recent years, it has been demanded to further decrease the weight and pressure loss of a ceramic honeycomb structure which serves as a catalyst carrier for automotive use, and it has been attempted to decrease the thickness of the cell walls to meet the demand. However, a decrease in the cell wall thickness results in a decrease in the strength of the honeycomb structure which may then be cracked since it can no longer withstand the load when it is being incorporated in a converter. Therefore, it was not possible to decrease the thickness of the cell walls to less than 100 .mu.m. To prevent a drop in the strength, it can be contrived to either maintain the strength by increasing the density of cordierite that constitutes the honeycomb structure or reinforce the strength relying upon the structure. In order to decrease the thickness by using the same cordierite starting materials as the one traditionally used without changing the coating property of the catalyst, however, it becomes necessary to reinforce the strength relying upon the structure.
As technology for reinforcing the ceramic honeycomb structure, Japanese Unexamined Patent Publication (Kokai) No. 49-88908 discloses a structure for enhancing the strength by forming the outer circumferential skin layer A thicker relative to the cell walls B, as shown in FIG. 1. However, an increase in the thickness of the outer circumferential skin layer A results in a decrease in the moldability of the ceramic honeycomb structure and in the resistance against thermal shock causing the probability of developing cracks due to heat at the time of cleaning exhaust gases. According to a honeycomb structure disclosed in Japanese Unexamined Patent Publication (Kokai) No. 51-155741, the strength is increased by increasing the thickness of the cell walls B' over a range of .+-.22 degrees in the diagonal directions, as shown in FIG. 2. According to this structure, however, thick portions are formed over increased areas, and the cell walls become thick at the central portions where the exhaust gases flow in large quantities, offsetting the merit of decreasing the thickness of the cell walls.
As described above, according to the conventional structures, it is difficult to maintain the strength of the structure while decreasing the thickness of the cell walls to smaller than 100 .mu.m without decreasing the moldability or the thermal shock resistance. The object of the present invention is therefore to provide a ceramic honeycomb structure which compensates for a decrease in the strength derived by reduction in thickness of the cell walls, sufficiently withstands the load at the time of being incorporated in a catalytic converter, and exhibits favorable moldability and thermal shock resistance. The invention further provides an extrusion die used for producing the ceramic honeycomb structure.